Ecology and Oceanography in the Gulf of Mexico The Dead Zone Ecology and Oceanography in the Gulf of Mexico

Part I – The Problem Sue’s Dad has to travel further out into the gulf to fish. Why do the fish disappear near the Louisiana shore in the summer? Sue visits a Biology Professor for help Dead Zone: dissolved oxygen levels in the water get so depleted the water cannot sustain life.

Mississippi River Drainage Basin

Detail of the Dead Zone in the Gulf of Mexico Red shows lowest O2 levels

Part II – What affects the Dissolved Oxygen? Paula predicts a physical cause Conditions that limit amount of oxygen that can be dissolved from the atmosphere  Temperature  Salinity  pressure Conditions that facilitate the transfer of oxygen across the air-surface interface Boats Waves Wind Conditions that are the result of advection (transfer of a property – by mass movement of water) Ocean currents Upwelling/downwelling River outflow

Part II – What affects the Dissolved Oxygen? Zack predicts a biological cause Photosynthesis by producers Adds oxygen Cellular Respiration Plants &animals Uses oxygen

Fluctuation over the seasons Physical influences Temp changes seasonally Mississippi discharge peaks in the spring Storms and strong winds are more common in the autumn and winter Biological influences Reactions occur faster in warmer temperatures Day length increases in spring through mid-June Availability of food or nutrients

Part III – How do the Gulf Waters Change with the Seasons? Temperature of deepwater does change, but not as dramatically as the surface water Huge volume of freshwater that enters the Gulf via the Mississippi River in April due to snow melt and spring rains Mississippi discharge is lower in September and the fall winds and storms agitate and mix the water. Hypoxic layer increases in size extending up to 12 meters

Let’s Talk Water – Required for Life Unique Structure Gives rise to some unique properties For this study: a universal solvent Ex. Salt (NaCl) Density Relationships of H2O Temp vs. Density – Inverse relationship* *as long as temp stays above 4C

What About Dissolved Oxygen? As terrestrial organisms we need O2 from air Aquatic organisms need O2 also! How? Dissolved Oxygen Microscopic bubbles of O2 gas Amount of D.O. varies Depends on physical and chemical make up of water

Marine Food Web